A promo is a teaser comic for a series.
What are Promos?Edit
Promos are usually one panel strips.It contains a little bit of information for the new series you are going to make without giving away too much about the ending. It's similar to the cover page of a book, only it's for a series of comics.
Why are Promos made?Edit
Promos inform the reader what the series is about.If the series is a fantasy story,and the reader likes humor strips,the reader knows that this series is not good for them.It is also like an advertisement for the series. Sometimes people will make promos for series they won't make.